Process Monitoring the Processing of a Material

ABSTRACT

The invention relates to a method for monitoring a processing region of a workpiece ( 10 ) on which laser processing is being carried out, in which method the radiation emitted by the processing region ( 11 ) is detected by a detector system ( 22 ) in a space-resolved manner, wherein the radiation of the processing region ( 11 ) is detected for each elemental area of the processing region ( 11 ) imaged onto the detector system at least two wavelengths simultaneously. Accurate process monitoring may thereby be carried out.

The invention relates to a method for monitoring a processing region ofa workpiece on which laser processing is being carried out, in whichmethod the radiation emitted by the processing region is detected by adetector system in a space-resolved manner, and to an apparatus formonitoring laser processing of a workpiece, having a filter arrangementand a detector system.

In laser processing procedures it is often desirable and even necessaryto ascertain the temperatures within a predefined processing regionduring the processing operation in order, for example, to be able tomake inferences regarding the particular quality to be adhered to duringthe processing operation and the adherence to specific processingregions, but also for the feedback control for the processing operation.

Since during the processing operation the temperatures at the differentpositions of a respective processing region vary locally and over timein dependence on a number of parameters such as, for example, therespective laser output power used in the processing operation, thedeflection speed and/or direction of laser beams, and the respectiveworkpiece to be processed, space-resolved determination of temperatureswithin a processing region is desirable.

Known systems measure and assess the welding process with individualphotodiodes or CCD/CMOS cameras. In that method, infrared radiation iscaptured in the form of a simple radiation intensity measurement. In thecase of those radiation measurements, inter alia absolute temperature,emission coefficient, illumination of the measurement field etc. aresuperposed. Furthermore, radiation intensity measurements are highlydependent on fouling of the optical systems, smoke and plasma in theobservation beam path. Assessment of a process using intensitymeasurements is therefore inaccurate.

An arrangement for space-resolved temperature measurement is known fromDE 10 2004 051 876 A1, wherein a predefinable processing region of aworkpiece can be imaged onto an optical detector measuring in aspace-resolved manner, and at least one optical filter that blocks theelectromagnetic radiation of one or more laser processing beams isdisposed in the beam path between the processing region and the opticaldetector.

The object of the present invention is to provide a method and anapparatus with which improved quality control is possible.

That object is attained by a method of the kind mentioned in theintroduction, in which the radiation of the processing region isdetected for each elemental area of the processing region imaged ontothe detector system at least two wavelengths simultaneously. The dataobtained for two wavelengths may be analyzed and set in relation to eachother. The processing region comprises, in the sense of the invention,the keyhole, the molten film surface thereof, the surrounding weld poolproduced in laser welding, the wake of a weld pool, the solidifiedmaterial and a surrounding heat-affected zone. By suitable analysis itis possible to calculate temperature gradients in the wake of the weldpool, in the solidified molten material and in the heat-affected zone.It is thereby possible, for example, to characterize the dissipation ofheat into the workpiece. In addition, internal weld seam faults, such aslack of fusion, may be detected. It is also possible to determine theinfluence of the material, such as, for example, hardening. It isfurthermore possible to ascertain the actual weld pool dimensions on thebasis of the phase transitions from molten to solid. Ultimately, acharacterization of the coupling of energy into the material and hencean inference as to internal weld seam characteristics, such asvariations in the penetration depth, lack of fusion at the lap jointetc., is possible. The method is therefore suitable for monitoring theprocess and especially the quality of the processing procedure and theweld seam produced.

In an especially preferred variant of the method, it may be providedthat a temperature, especially an absolute temperature, and/or anemission coefficient is/are determined from the detected radiation foreach elemental area detected by the detector system. The determinationof the absolute temperature and the emission coefficient from theradiation measured at least two wavelengths may be performed byproportional radiation pyrometry methods known per se and/or byanalytically fitting the electromagnetic radiation to physical radiationlaws. For this, the radiation of each detected elemental area on theworkpiece is converted by means of analytical separation into anabsolute temperature and an emission coefficient. An image oftemperature values of the keyhole, its molten film surface, the moltenmaterial surrounding the keyhole, the wake of the weld pool, thesolidified material and the heat-affected zone is thus obtained, withthe additional, unequivocal information of the phase transition frommolten to solid.

It is especially preferred if, based on the temperature, especially theabsolute temperature, and/or the emission coefficient, a qualityassessment is carried out. The method according to the invention is moreaccurate than known methods. For example, a molten zone on which anoxide layer has already formed can be analyzed more accurately. Owing tothe oxide layer, there is a higher emission coefficient in that zonecompared with the molten material, while both zones—with and withoutoxide layer—exhibit similar temperatures. In the case of known methods,such a region is wrongly detected as solidified molten material, sincethe analysis of the phase transition from molten to solid is based on achange in radiation intensity brought about by the different emissivity.By separate analysis of the temperature and the emission coefficient, ajump in the emission coefficient but only a slight change in theabsolute temperature will occur at such a site. The contour and the sizeof the molten phase can be ascertained by determining the regions inwhich the absolute temperature is below the melting point of thematerial. Furthermore, the effect of the temperature and the emissioncoefficient having an opposite effect to each other is compensated for.A high absolute temperature at a low emission coefficient wronglyresults in lower measured values for radiation intensity or, in thecamera image, in darker representation of an open weld pool in contrastto the solid material. Accordingly, in the case of the known, simpleradiation intensity measurement, a lower measured temperature of a weldpool in contrast to the solidified or solid material is wronglydetermined.

In addition to the detection of the actual phase transition from moltento solid, absolute temperatures also afford better means of assessmentfor the solidified material and the heat-affected zone. Temperaturegradients over time and/or space may be ascertained here with greataccuracy and may be used to express the quality of the welded joint.

In one advantageous variant of the method, it may be provided that theradiation emitted by the processing region is split into a plurality ofindividual beam paths and the individual beam paths are filtered. It isthereby possible for the radiation radiated from a elemental area to bedetected and analyzed at differing wavelengths. The splitting of theradiation ascertained may be performed, for example, with a beamsplitter. Alternatively, the radiation may be coupled into fiber opticsthat are associated with different elemental areas and at the ends ofwhich a wavelength-selective detection of the radiation takes place.

In an especially simple manner, the radiation emitted by a elementalarea may be detected at least two wavelengths if optical filtering forat least two wavelengths is carried out for each elemental area.

Further advantages of the invention are obtained when a plurality ofwavelength-dependent images of the processing region are produced. Fromthose thermographic images of the process it is possible to ascertainthe actual temperature image and an emission coefficient image. It isthus possible for the temperature distribution in the processing regionand the emission coefficient distribution in the processing region to berepresented graphically and evaluated in a simple manner.

The scope of the invention also includes an apparatus of the kindmentioned in the introduction, wherein at least two sensor elements eachhaving an optical filter associated therewith are associated with eachimaged elemental area, and wherein the filters filter the emittedradiation at different wavelengths. The same filter may be associatedwith a plurality of sensor elements—for various elemental areas. It isalso possible, however, for each sensor element to have its own filter.An apparatus of that kind is suitable for carrying out the method of theinvention, so that the advantages mentioned in relation to the methodmay be obtained. Preferably, the apparatus according to the inventionforms a unit with a laser beam welding apparatus. It is thereby possiblefor the analysis and quality control to be performed simultaneously withlaser processing of the material. A short interval in time or spacebetween the welding process and the process monitoring operation may betolerated here.

In order to be able to perform the analysis of the radiation recorded atdifferent wavelengths it is advantageous for an evaluating device to beprovided for determining a temperature value and/or an emissioncoefficient for each elemental area.

In a preferred embodiment, a beam splitter may be provided in the beampath. By means of a beam splitter, the radiation radiated from theprocessing region may be split into a plurality of individual beam pathsand imaged onto a suitable detector system by optical imaging systems inselective, narrow-band spectral ranges. In particular, in that operationa plurality of images may be produced on one detector. From thosethermographic images of the process it is possible to calculate atemperature image and an image of the emission coefficients.

As the detector system, a matrix camera, for example, may be provided.Preferably, the matrix camera has a spectral sensitivity that issuitable for the temperature radiation to be measured, that is, has asufficiently high spectral sensitivity. Alternatively, a plurality ofindividual cameras, especially matrix cameras, may be provided as thedetector system, with a single, selective spectral range being imagedonto each individual camera. The or at least one matrix camera may bemade from different semiconductor materials.

Suitable cameras are detectors for the radiation range from the visualspectral range through near-infrared to far-infrared. Suitable camerasof the detector system are, for example, CCD, CMOS and/or InGaAscameras, but that list is by no means exhaustive and other types ofcamera may be used. When individual cameras are used it is also possiblefor a plurality of different cameras to be combined. For example, fordifferent spectral ranges to be measured, different cameras with variousspectral sensitivities may be used.

In a preferred embodiment of the invention, it may be provided that thedetector system comprises a plurality of photodiode arrays. Thephotodiode arrays may comprise one or more photodiodes. The photodiodearrays may be in the form of spectrally narrow-band single or multiplediodes so that the appropriate radiation component may be detected foreach elemental area in the processing region simultaneously. Themultiple or single diodes may be made from different semiconductormaterials.

In one advantageous embodiment, fiber optics and/or optical imagingsystems may be arranged in the beam path in front of the photodiodearrays. The photodiode arrays may form a matrix sensor. The photodiodesor fiber optics, and the images of the elemental areas of the workpiece,may be in any desired arrangement relative to one another.

By means of the filter arrangement, the radiation of each elemental areadetected may be filtered at least two wavelengths, so that two imagepoints having differing spectral radiation information are produced fromeach elemental area. The filter arrangement may be integrated in thecamera matrix, arranged on the matrix surface in front of the individualsensor elements or provided between the beam splitter and the detectorsystem. Furthermore, the filter arrangement may be integrated in thebeam splitter mirror or arranged on or integrated in the beam splitter.

It is furthermore conceivable for the filter arrangement to comprise achessboard pattern, a striped pattern or an arrangement of opticalsingle filter elements. In particular, the filter arrangement may besuitably selected according to the detector system used or the localtemperature measurement range required. In other words, the wavelengthsof the filters of the filter arrangement may be matched to thetemperature range that is to be measured. Larger filters are preferablyused in combination with matrix cameras. The radiation of the totalprocessing region is split into a plurality of beam paths by a beamsplitter in a spectrally neutral or spectrally selective manner. In thatprocedure, each beam path is passed to a wavelength filter disposedbehind the beam splitter and in front of or on the matrix arrangement.Alternatively, the optical filter may be mounted on the beam splitter orthe beam splitter mirror itself. Thus, the radiation of the totalprocessing region is filtered. If individual diodes are used, a smallfilter element may be associated with each diode and hence with eachelemental area of the processing region, so that only the radiationemitted by one elemental area is filtered by the filter element.

The measuring arrangement is calibrated with a reference radiationsource. There is preferably used for that purpose a full radiator havinga homogeneous temperature and a homogeneous temperature distributionover its reference radiation surface. The reference radiation is firstimaged on the detectors by means of the apparatus/optical systememployed and then calibration of each matrix/detector element isperformed.

Further features and advantages of the invention will be apparent fromthe following description of illustrative embodiments of the inventionwith reference to the Figures of the drawings which show inventivelysignificant details, and from the claims. The individual features mayeach be implemented individually or a plurality thereof may beimplemented in any desired combination in a variant of the invention.

Preferred illustrative embodiments of the invention are shownschematically in the drawings and will be described in detailhereinafter with reference to the Figures of the drawings, in which:

FIG. 1 is a first schematic illustration of a first apparatus forquality control of laser processing;

FIG. 2 is a second schematic illustration of a second apparatus forquality control of laser processing.

FIG. 1 shows a workpiece 10 which is being processed with a laser beam48. In a processing region 11 of the workpiece 10, a weld pool 12 formswhich emits electromagnetic radiation 13. The electromagnetic radiation13 passes via an optical focusing system 14 onto a beam splitter mirror15 and to a beam splitter 16 which may be in the form of a prism.

As shown in FIGS. 1 and 2, the beam splitter mirror 15 may be configuredto be highly transmissive for the laser radiation 48 and reflective forthe wavelength ranges for thermography. Other variants are possible,such as an arrangement in which the beam splitter mirror 15 isconfigured to be highly reflective for the laser radiation and highlytransmissive for the wavelength ranges for thermography.

The beam splitter 16 splits the electromagnetic radiation 13 into aplurality of beam paths 17, 18. The beam paths 17, 18 pass via arespective imaging optics 19, 20 in the form of an optical focusingsystem and via a filter arrangement 21 onto a detector system 22 in theform of a matrix camera. The optical focusing system may comprise a lensand/or a mirror. The imaging optics 19, 20 are configured in such amanner as to compensate for image distortion due to chromaticaberration. In addition, especially for axial and lateral positioning,the optics 19, 20 may be configured to be displaceable together in thebeam direction and laterally and also to be adjustable relative to eachother.

The filter arrangement 21 comprises a first filter 23 for filtering afirst wavelength and a second filter 24 for filtering a secondwavelength. By means of filtering, therefore, a spectral selection takesplace and two different radiation images 25, 26 of the processing region11 are produced on the detector system 22. From those radiation images25, 26 it is possible to ascertain by analytical methods an absolutetemperature and an emission coefficient for each elemental area of theprocessing region 11. The analysis is performed in an evaluating device30. The absolute temperature and/or the emission coefficient provide inturn information on the quality of the processing operation.

As shown in FIGS. 1 and 2, a processing region 11, which comprises thekeyhole, the weld pool, the solidified molten material and theheat-affected zone, is divided into a plurality of elemental areas 35.According to FIG. 2, the radiation emitted by the elemental areas 35passes via an optical focusing system 14 onto a beam splitter mirror 15.Arranged in the beam path of the electromagnetic radiation 13 is animaging optic 36 which couples the radiation of each individualelemental area 35 into fiber optics 37, 38 held at one end by a holder50. The radiation passes via the fiber optics 37, 38 to filter elements39-42, with two filter elements 39-42 being associated with eachelemental area 35 and hence with each fiber optic 37, 38. The radiationfiltered by the filter elements 39-42 is detected by sensor elementsformed by photodiodes 43-46 of a photodiode array 47. Thus, once again,different spectrally selective radiation measurement points, which maybe analyzed by an evaluating unit 30, are obtained for each elementalarea 35. In particular, a temperature and/or an emission coefficient maybe determined for each elemental area 35. The number of elemental areas35 analyzed is determined by the number of sensor elements. Thephotodiode array 47 forms the sensor system.

The optical fibers 37 and the image points 35 imaged may be imaged inthe form of a matrix or any other desired arrangement (circle, line,cross etc.) relative to one another on the workpiece.

1. A method for monitoring a processing region of a workpiece on whichlaser processing is being carried out, the method comprising: detectingthe radiation emitted by the processing region in a space-resolvedmanner, wherein the radiation of the processing region is detected for aplurality of elemental areas of the processing region at least twowavelengths simultaneously; and determining an emission coefficient fromthe detected radiation for each of the plurality of elemental areas. 2.(canceled)
 3. The method of claim 1, further comprising performing aquality assessment based on the emission coefficient.
 4. (canceled) 5.The method of claim 1, further comprising optically filtering at leasttwo wavelengths for each of the plurality of elemental area.
 6. Themethod of claim 1, further comprising producing a plurality ofwavelength-dependent images of the processing region. 7-9. (canceled)10. The apparatus of claim 24, wherein the detector system includes atleast one matrix camera.
 11. The apparatus of claim 24, wherein thedetector system includes a plurality of individual cameras.
 12. Theapparatus of claim 10, wherein the at least one matrix camera includesdifferent semiconductor materials.
 13. The apparatus of claim 24,wherein the detector system comprises a plurality of photodiode arrays.14. (canceled)
 15. The apparatus of claim 24, further comprising atleast one of optical fibers and imaging optics arranged in a beam pathin front of the sensor elements.
 16. The apparatus of claim 15, whereinthe optical fibers are arranged as a matrix or in any other desiredgeometric shape relative to one another.
 17. The apparatus of claim 13,wherein the photodiodes of the plurality of photodiode arrays includedifferent semiconductor materials.
 18. The apparatus of claim 24,wherein the filter arrangement is one of integrated in a camera matrixof the detector system, arranged on a matrix surface positioned in frontof the individual sensor elements, and provided between a beam splitterand the detector system.
 19. The apparatus of claim 24, wherein thefilter arrangement is integrated in a beam splitter mirror.
 20. Theapparatus of claim 24, wherein the filter arrangement comprises anarrangement of the optical filters in a chessboard pattern, a stripedpattern or an arrangement of optical single filter elements. 21-22.(canceled)
 23. The apparatus of claim 15, wherein the imaging optics areconfigured to be adjustable relative to each other for at least one ofcompensating for chromatic aberration and axial and lateral positioning.24. An apparatus for monitoring laser processing of a workpiece, theapparatus comprising: a detector system with sensor elements fordetecting radiation emitted from a plurality of elemental areas, whereinradiation emitted from one of the elemental areas is detected by atleast two of the sensor elements; a filter arrangement with opticalfilters, wherein at least one of the optical filters is associated withat least one of the sensor elements, wherein optical filters associatedwith sensor elements detecting radiation emitted from the same elementalarea filter the radiation at differing wavelengths; and an evaluationdevice for determining at least an emission coefficient for theplurality of elemental areas based on the detected radiation.
 25. Theapparatus of claim 24, wherein the sensor elements are arranged as amatrix or in any other desired geometric shape relative to one another.26. The apparatus of claim 24, wherein the filter arrangement is mountedon or integrated in a beam splitter.
 27. The apparatus of claim 15,wherein the at least one of optical fibers and imaging optics areconfigured to produce images of the elemental areas that are arranged asa matrix or in any other desired geometric shape relative to oneanother.